#include<bits/stdc++.h>
#define ONLINE_JUDGE
#define ll long long
#define ull unsigned long long
using namespace std;

const int N = 1e6 + 10, INF = 0x3f3f3f3f;

int n;
string s;
stack<char> st;
map<char, char> hs;

void solve()
{
    for (int i = 0;i < 13;i++)
        hs['a' + i] = 'z' - i;
    for (int i = 0;i < 13;i++)
        hs['Z' - i] = 'A' + i;

    cin >> n >> s;
    for (int i = 0;i < n;i++) {
        if (st.empty() || hs[st.top()] != s[i])
            st.push(s[i]);
        else st.pop();
    }
    cout << st.size();
}

signed main()
{
    // ios::sync_with_stdio(0);
    // cin.tie(0);cout.tie(0);
#ifndef ONLINE_JUDGE
    std::istringstream in(R"()");
    std::cin.rdbuf(in.rdbuf());
#endif
    int T = 1;
    //cin>>T;
    for (int i = 1;i <= T;i++) {
        solve();
    }
}